The theme of the poem is that love is difficult, but worth fighting for.
In this sonnet, the speaker is trying desperately to make up with his lover. She seems to be combative, and he is trying to tell her that they can call a truce. He is comparing love to a war, because figuratively the language is often the same. We “win” our lover’s hearts, for example.
I offer free Conditions of fair Peace!
My heart for hostage that it shall remain.
Love is all about compromises. It can seem like you are being held prisoner, when you are so in love that you can’t stand being away from the person. In this case, giving in is kind of a good thing.
I send Defiance! since if overthrown,
Thou vanquishing, the conquest is mine own!
As the speaker points out, by losing he is actually winning, because he gets what he wants—his lover.